This invention relates to a method and apparatus for flowing a stream of laminar flowing material into a mold having at least one runner branch, branching in at least two directions. More specifically, the present invention pertains to a method and apparatus for repositioning the non-symmetrical conditions of the flowing material to a desired position in a circumferential direction while maintaining continuity between laminates from the center through to the perimeter of the runner.
A conventional mold set for injection or transfer molding of laminar flowing polymer containing materials is constructed of high strength metals, usually tool steels having a very high compressive yield strength. A molded part is formed within a mold cavity. The mold cavity opens and closes during each molding cycle along a parting line in order to remove, or eject, the molded part. The material producing the molded part is fed from a material source to the cavity through a runner system. Often, several spaced mold cavities are defined in the mold. These cavities are each connected to a material source through a runner. Runners may include branches. A branch may occur at the end of a first runner section and would intersect at some angle relative to the first runner section. The angled branching second runner section may extend in one or more directions from its intersection with the first runner section. Non-symmetrical conditions are developed in a runner, flowing a stream of laminar flowing material, when a runner branch branches in at least two direction from the intersection with the first runner section. Branching may continue at the end of any number of progressively branching runner sections.
In multi-cavity molds, it is important that the material is delivered to each cavity of the mold at the same time and with the same pressure and temperature. Any variations in these conditions will result in variations in the parts which are produced within these cavities. Such variations can include the size, shape or weight of the product as well as the mechanical properties and cosmetic appearance of the product. To help assure balanced conditions, the length and diameter of the runner feeding each cavity in a multi-cavity mold is preferably kept the same. This usually results in the runners being laid out in either a radial pattern, a branching "H" pattern, or some combination of a radial and a branching "H" pattern. With the radial pattern, the melt travels radially outwardly from the material source directly feeding a single cavity. Variations of this may branch the end of each runner section and feed two or more cavities. With an "H" type pattern, the runner is continually split in two directions at the end of a given section. In some cases a radial pattern can be placed at the end of a branching "H" patterned runner.
When molding parts using multi-cavity molds, it is important that each cavity in the multi-cavity mold produce substantially identical parts. This results in consistent part quality and maximum productivity. In order to provide such a mold, the cavity dimensions must by nearly identical for each of the several cavities and the cooling and delivery of the flowing material to each cavity should be substantially the same. It is, therefore, standard practice in the design of multi-cavity molds to "naturally balance" the runner system in order to help provide the required mold filling consistency. In naturally balanced runners, the same cross sectional shape and length of runner feeds each cavity. The same concept of a natural, or geometrically, balanced runner system may also be applied to multiple runner branches which may be feeding a single part at multiple locations.
Most multi-cavity injection or transfer molds are designed with a naturally balanced or geometrically balanced runner system in order to minimize variations in the material flowing into the cavities during production.
Despite the geometrical balance, it has often been observed that the filling of molds utilizing these naturally balanced runner designs result in imbalances. In most case, such imbalances have not been recognized until there are more than four cavities in the mold. However, the imbalance is actually dependent on the number of branches in the runner and can even affect a part molded in a single multi-gated cavity, dependent on the layout of the runner system. It has been found that the parts formed in some of the cavities, usually those on the inside branches closest to the material source, are commonly larger and heavier than are the parts formed in the other cavities.
These flow imbalances have historically been attributed to variations in mold temperature and/or mold deflection. Applicant has identified that there is a flow-induced cavity filling imbalance which exists in many of the most commonly used and accepted "naturally balanced" runner designs such as geometrically balanced "H" and modified "H" patterned runners, especially those with eight or more cavities. The flow imbalance can be created by a non-symmetrical shear distribution within a laminar flowing material as it travels through the runner system. Flow imbalance can also be created in a runner channel when a laminar flow material has a non-symmetrical temperature distribution created either by localized shear or differences in temperature between the flowing material and the runner wall. Both of these non-symmetrical conditions can result in variations in the viscosity of the flowing material and, in some cases, in its structure. In most cases, during conventional molding of thermoplastic and thermosetting materials, the result is a high sheared hotter, lower viscosity material around the inner periphery of the runner channel surrounding a relatively low sheared cooler, higher viscosity material in the middle of the runner channel. As flow is laminar, when a branch in the runner occurs, the high sheared hotter material along the perimeter remains in its relative outer position while the inner material is split and is now positioned on the opposite side of the flow channel from the high sheared hotter material. This side to side variation will create a variation between upcoming side to side branching runners, or a mold cavity, where the high sheared hotter material will flow to one side and the low sheared cooler material will flow to the other side.
Attention in this regard is directed to the article by Beaumont and Young in the Journal of Injection Molding Technology, September 1997, Volume 1, No. 3 entitled "Mold Filling Imbalances in Geometrically Balanced Runner Systems" (pages 133-143). This article is incorporated herein by reference in its entirety.
The problem has become more evident in recent years as tolerances of molded plastic parts have become more demanding and attention to quality has increased. The trend toward the use of smaller diameter runners, which it was thought would improve the molding process, has compounded the problem. Attention is also directed to the article by Beaumont, Young and Jaworski entitled "Solving Mold Filling Imbalances in Multi-Cavity Injection Molds" found in the Journal of Injection Molding Technology, June 1998, Volume 2, No. 2, pages 47-58. This article is also incorporated herein by reference in its entirety.
The imbalance found in a multi-cavity mold can be significant, resulting in mass-volume, flow-rate variations between the cavities of as high as 19--1 in extreme cases. The magnitude of the imbalance is material-dependent as is the sensitivity of the imbalance to process. A variety of different types of thermoplastics, including amorphous and semi-crystalline engineering and commodity resins, have been shown to exhibit significant mold filling imbalances in branching runner molds.
While the majority of the description herein will refer to thermoplastic materials, it should be recognized that imbalanced conditions can occur in any mold with a branching runner, branching in at least two directions, in which a variety or types of fluid can flow. Such imbalances occur for any fluid exhibiting a) laminar flow and b) viscosity which is affected by shear rate (as with a non-Newtonian fluid) and/or by temperature c) characteristics where variations in shear or flow velocity across a flow channel will create variations in the materials characteristics. Both of these characteristics are typical of thermoplastics, thermosetting materials and many of today's powdered metal and powdered ceramic molding materials. A polymer carrier is often employed with powdered metals and powdered ceramics. It is the polymer which gives such powdered metal or powdered ceramic materials the same characteristics as plastic materials exhibit in regards to viscosity effects and laminar flow.
The traditional methods of balancing flow in multi-cavity molds by restricting high flow runner branches or gates, cannot be expected to provide both a pressure and a thermal balance in the flowing material. Even if a pressure balance can be achieved, a melt temperature variation between the several cavities remains. Additionally the balance achieved by this means is very sensitive to material and process changes.
The ability of this invention to control the position of the asymmetric material conditions not only can be used to balance flow in runner branches but can be used to control the asymmetric material conditions flowing into a part forming mold cavity. Many of the properties of the molded part can be influenced by conditions of the melt from which it is formed. Some of these include how the molded part will shrink, warp, its mechanical properties and its appearance. With an understanding that a part might warp as a result of temperature variations, the asymmetric temperature across the laminar flowing material entering the cavity, through a runner and gate, could be positioned to control this warpage. With thermoplastic materials which will commonly warp towards a hot side of a mold, the asymmetric laminar flowing material could be positioned such that the hotter melt entering the cavity be placed along the cooler mold half. This could potentially offset the mold temperature variations. A similar principle could be applied to address effects of part geometry on warpage or some other need to control distribution of other material properties which might be affected by the shear and temperature variations.
Flow diverters have been used to change the flow patterns in laminar flowing material. One known device of this type is illustrated in U.S. Pat. No. 5,683,731 of Deardurff et al. This device contains a central flow channel and a plurality of diverters. The device is positioned in a melt stream. Melt from some portion of the inner laminates of the melt stream is fed into a central flow channel and melt from some portion of the outer laminates of the melt stream is fed into a plurality of diverters which are adjacent to the central flow channel. The melts from the two flow paths are later recombined such that the material from each of these flow regions is distributed equally between the plurality of flow channels.
However, in Deardurff et al., the inner and outer laminates of the flow channel are separated and recombined. This results in a more complicated and expensive device than what is necessary. Moreover, Dearduff's device would not be practical in a runner system which solidifies and is ejected from the molding process during each cycle as the device would become molded into the runner and ejected from the mold. Therefore, Deardurff's devise is limited to hot runner, or non solidifying runner, applications where the plastic in the runner does not solidify and is not ejected from the mold.
In addition, Deardurff's device is relatively complex and requires consideration of the relative sizes and shapes of the central flow channel and the diverter. The sizes and shapes of these channels will dictate a) how much of the outer laminates will be repositioned relative to the central flow channel b) where they will be positioned and c) their distribution relative to each other. Also, any changes in material or process may alter the distribution of the melt between the central and diverter channels. Furthermore, Deardurff's device accomplishes its objective by selectively diverting some portion of the outer laminates and distributing them among a plurality of channels. The disadvantage of this design is that it can only selectively rearrange the melt across the flow channel in two distinct inner and outer regions. This limits the contribution of this devise as the variation across a melt channel is continuous and complicated by the fact that the change in the materials conditions across the flow channel are not normally linear. Achieving a continuous redistribution of melt is not possible with a device which selectively separates the laminates into two distinct regions, namely the inner and outer regions.
Other known diverters are equally disadvantageous. None of these devices is capable of repositioning the laminates in a melt in a circumferential direction while maintaining continuity between the laminates in a radial direction.
Additionally, the division of the flow channel into a multitude of flow channels, i.e. the central channel and the several diverter channels in the known devices creates a potentially significant pressure loss--as pressure drop is approximately a function of the radius of a round flow channel to its fourth power--due to the resultant smaller channels. The alternative is to significantly increase the cross section of all the flow channels in order to alleviate the high pressure loss resulting from the smaller flow channels which significantly complicates the construction of such a mold.
Accordingly, it has been considered desirable to develop a new and improved process and apparatus for controlling flow in runners which would overcome the foregoing shortcomings and others while providing better and more advantages overall results.